Plate tectonics Flashcards
(28 cards)
What are the layers of Earth’s structure (innermost to outermost)?
Inner core (solid iron/nickel)
Outer core (liquid iron/nickel)
Mantle (solid silicate rocks, ductile asthenosphere)
Crust (rigid lithosphere: oceanic ~3.2 g/cm³, continental ~2.8 g/cm³)
How do P-waves and S-waves differ in behavior?
P-waves: Compressional, travel through solids/liquids (~6–8 km/s).
S-waves: Shear, only through solids (~4–5 km/s). Liquid outer core blocks S-waves!
What is the Mohorovičić (Moho) discontinuity?
Boundary between crust and mantle where seismic wave velocity increases sharply.
Define isostasy and give an example.
Buoyant equilibrium of crust “floating” on mantle (e.g., mountain roots). Example: Scandinavia rising post-glacial ice melt.
What causes Earth’s magnetic field?
Convection of liquid iron in the outer core (geodynamo effect).
What are magnetic reversals?
Periodic flips in Earth’s magnetic polarity (e.g., last reversal ~700,000 years ago). Recorded in ocean crust basalt stripes.
What evidence supports continental drift?
Continental jigsaw fit (Pangea).
Matching rock assemblages (e.g., Drakensberg basalt/Antarctica).
Fossil distributions.
Describe seafloor spreading.
New crust forms at mid-ocean ridges (divergent boundaries).
Magma cools, records magnetic polarity → symmetric stripes (Vine-Matthews hypothesis).
Oldest ocean crust: ~200 Ma (vs. continents ~4 Ga).
What are the three plate boundary types?
Divergent (e.g., Mid-Atlantic Ridge).
Convergent (e.g., Himalayas, subduction zones).
Transform (e.g., San Andreas Fault).
Compare ocean-ocean vs. continent-continent convergent boundaries.
Ocean-ocean: Subduction → island arcs (Japan).
Continent-continent: Collision → mountains (Himalayas). No subduction!
What drives plate tectonics?
Mantle convection (asthenosphere as conveyor belt).
Ridge push (sliding off elevated ridges).
Slab pull (subducting plate drags rest).
Define geotherm.
The increase in temperature with depth in Earth’s interior (~25°C/km in crust).
What are the types of faults?
Normal: Extensional stress (hanging wall drops).
Reverse: Compressional stress (hanging wall rises).
Strike-slip: Shear stress (horizontal motion, e.g., San Andreas).
What is epidote?
A green metamorphic mineral common in metabasites (altered mafic rocks).
What is hotspot volcanism?
Fixed mantle plumes melting crust (e.g., Hawaii, Yellowstone).
Tracks show plate motion (e.g., Snake River Plain).
What is the asthenosphere?
Ductile upper mantle layer enabling plate movement (convection occurs here).
Why are there no earthquakes deeper than 700 km?
Rocks become ductile (not brittle) in lower mantle → no faulting.
What is shear stress?
Stress parallel to a surface (causes strike-slip faults).
What is the Ring of Fire?
A zone of frequent earthquakes/volcanoes around the Pacific Ocean, caused by subduction of oceanic plates under continental plates.
How does subduction trigger volcanoes?
Oceanic crust melts as it sinks into the mantle, releasing water that lowers the melting point of surrounding rock → magma rises to form volcanoes (e.g., Andes).
Why is oceanic crust younger than continental crust?
Oceanic crust is continuously recycled at subduction zones (max age ~200 Ma), while continental crust is buoyant and preserved.
What is slab pull vs. ridge push?
Slab pull: Subducting plate’s weight drags the rest of the plate (strongest force).
Ridge push: Gravity slides plates off elevated mid-ocean ridges.
What causes transform boundaries?
Shear stress from plates moving past each other horizontally (e.g., San Andreas Fault). No crust is created/destroyed.
How do magnetic stripes on the seafloor prove plate tectonics?
Symmetric patterns of normal/reversed polarity basalt stripes record seafloor spreading at mid-ocean ridges (Vine-Matthews hypothesis).
